Hallberg began his senior career at Kalmar FF, then in 2014, he signed for Italian club Udinese, and during his time there went on loan to Vålerenga, Hammarby IF, Ascoli and Kalmar FF. He signed for Vejle BK in 2018, where he spent one season. Hallberg then played in three seasons for Scottish club Hibernian. He made his full international debut for the Sweden national team in 2016, and has won a total of three caps and scored one goal.
Career
Early career
Hallberg grew up in the small town of Ljungbyholm, shortly to the south of Kalmar. There he started with the local club Möre BK where he played for the first team in the seventh tier of Swedish football at age thirteen.[2]
In 2010 Hallberg was brought into the Kalmar FF youth section. Two years later he became the youngest-ever first-team player for the club when he made his Allsvenskan debut at the age of sixteen.[3] On July 15, 2012, Hallberg became the fourth-youngest Allsvenskan goalscorer of all time when he scored the game-winning 2–1 goal against IF Elfsborg.[4] In December 2012 he signed a new four-year contract extension.[3] After his second season in Allsvenskan as a seventeen-year-old, Hallberg was ranked as the 14th-best player in the league by newspaper Expressen.[5]
Udinese
In August 2014, Hallberg signed for Italian club Udinese. He made his debut in January 2015.
Hallberg joined Danish club Vejle Boldklub in the summer 2018. He left the club at the end of the 2018–19 season.[6]
Hibernian
Hallberg signed a three-year contract with Scottish Premiership club Hibernian in August 2019.[7] He scored his first goal for the club in a Scottish League Cup tie against Celtic on 2 November 2019.[8] He was released from his contract with Hibernian in January 2022.[9]
St Johnstone
Following his release by Hibernian, Hallberg signed an 18-month contract with St Johnstone in January 2022.[10] After prolonged renegotiations, Hallberg left The Saints in July 2023.[11]
Return to Kalmar
In August 2023, Hallberg returned to his first club Kalmar FF on a contract until the end of the season.[12]
International career
Hallberg has featured regularly at all age levels of Swedish youth national teams. In August 2013 he got selected for the first time ever to the Swedish U21 squad in a friendly against Norway.[13] He was selected for Sweden squads in January 2016[14] and January 2019.[15]